On account of a recent tendency that an optical communication system is small-sized and used for a higher bit rate transmission, engineers in this field place their hopes on the development of an optical communication module having a compact size and suited for a high bit rate communication. Especially, a module having an integrated circuit with the function of a signal amplification within can operate with a low driving voltage, makes an amplifier provided for a transmitting panel unnecessary, and contributes toward small-sizing a transmitter and reducing a cost thereof, hence efforts for developing the optical communication module are actively made in various research institutions. Hitherto, a simple semiconductor laser has been used as a light source in an optical communication module, and thereby an electrical signal has been converted into an optical signal by a direct modulation system. However, with the advance of technology in the optical communication that a transmission distance becomes longer and a bit rate of the communication becomes higher, the disadvantage of the direct modulation system is actualized. That is to say, according to the direct modulation system using a semiconductor laser diode, chirping in a wavelength of an optical signal arises at the time of modulation, and the waveform of the optical signal propagating through an optical is deformed. This phenomenon becomes noticeable as the bit rate of the signal transmission becomes higher and the transmission distance becomes longer. This problem is serious especially in the transmission system using the existing 1.3 .mu.m zero dispersion fiber. Even if the light source of 1.55 .mu.m band corresponding to a lower transmission lose is used in order to extend the transmission distance, the transmission distance is limited because of the dispersion distortion caused by chirping.
This problems can be settled by adopting an external modulation system in which the semiconductor laser is activated at a constant optical output power and the light emitted therefrom is modulated by a modulator separated from the semiconductor laser. Then, the external optical modulator is being actively developed. At present, the optical modulators of two types are regarded as promising for this purpose, the one is a dielectric optical modulator using dielectric material, such as LiNbO.sub.3 and etc., and the other one is a semiconductor optical modulator using a semiconductor, such as InP, GaAs and etc. In the aforementioned optical modulators, the semiconductor optical modulator can be integrated with the semiconductor laser, an optical amplifier, and other optical devices, and easily fabricated so that it is small seized and operates with a low driving voltage. Then, the engineers in this field place their hopes on the semiconductor optical modulator. Then, two types of the semiconductor optical modulator are worth notice. The one is an electroabsorption optical modulator, which intensity modulates a light by applying an electric field to a semiconductor and controls a light absorption coefficient thereof. In this case, an absorption edge of the light in the semiconductor is shifted to a lower wavelength region by the electric field on account of the Franz-Keldisch effect of a bulk semiconductor or the quantum confinement Stark effect in a multiple quantum well. The other one is a Mach-Zehnder type modulator, which make use of change of a refractive index of the semiconductor caused by an electrooptics effect (Pockels effect) of bulk semiconductor or the quantum confinement Stark effect. Although the Mach-Zehnder type modulator has a structure of interference type and cannot be formed into a simple linear waveguide structure, and methods for fabricating and driving it becomes complicated. On the other hand, the absorption optical modulator can remarkably reduce chirping of the wavelength as compared with the direct modulation system using the semiconductor laser, and can be monolithically integrated with the semiconductor laser without a great difficulty. So that, the results of research and development on the absorption optical modulator to be used as a light source in a transmitter module are reported by various research institutions in recent years.
On the aforementioned optical communication module which comprises an electroabsorption modulator integrated semiconductor laser light source and a built-in signal amplifier, results of developments are reported by Nakamoto et al. in the 23rd European Conference on Optical Communication, Vol. 1, pp. 7 to 10 (1997), and Doi et al. in Proceeding of General Conference of IEICE, p. 195, C-12-67 (1998). Moreover, Nishino et al. report the result of the research on the same subject on Technical Report of IEICE, CS94-23, OCS94-13, pp. 87 to 92 (1994), and Mineo et al. report the results of improvement of the characteristic of the optical communication module on Proceeding of General Conference of IEICE, C-214, p. 214 (1994). FIG. 1 shows the outline of the optical communication module developed by Nishino, Mineo et al.. As shown in FIG. 1, a semiconductor laser unit 302 composed of a light source and a modulator which are monolithically unified is mounted on a module casing 301. A signal light which is emitted from the laser unit 302 and modulated by a modulator therein is outputted through a transparent optical output port 303 formed on a part of the module casing 301. Moreover, a monitor unit 304 formed of a photodiode is situated at the back of the laser unit 302. A high frequency connector 305 serving as an input terminal is provided for the module casing 301 opposite to the optical output port 303. An amplifier 306 for amplifying a signal supplied from the high frequency connector 305 is set close to the high frequency connector 305. The amplifier 306 is connected with the laser unit 302 via a co-planar line 307 forming a signal line.
However, in the aforementioned optical communication module, it is necessary to lay the co-planar line 307 for electrically connecting the laser unit 302 with the amplifier 306 taking a long way around the monitor unit 304 situated at the back of the laser unit 302. Moreover, since the laser unit 302 is situated near the optical output port 303 in order to optically couple therewith in most cases, the co-planar line 307 which electrically connects the high frequency connector 305 and the amplifier 306, both being opposed to the optical output port 303, with the laser unit 302 becomes considerably long in conformity with the dimension of the module casing 301. Accordingly, the co-planar line 307 laid between the amplifier 306 and the laser unit 302 is long and comprises bent portions, hence it is difficult to maintain a satisfactory high frequency characteristic, and a tolerance in a fabricating process becomes severe. Moreover, since independent co-planar lines are necessary also, it becomes difficult to reduce the number of parts and fabricate the compact optical communication module because of increased length of the co-planar lines. As mentioned in the above, since the optical output port 303, the laser unit 302, the monitor unit 304 and the high frequency connector 305 are arranged nearly along a straight line, and especially the high frequency connector 305 and the optical output port 303 are arranged on a center line thereof, the conventional optical communication module is advantageous for an actual handling. However, as mentioned in the above, the conventional optical communication module is apt to cause troubles in a treatment of the high frequency signal, and cannot show satisfactory performances in operation in the high frequency region.